Switching amplifiers are commonly referred to as digital amplifiers, and may be implemented by either digital or analog means. Such switching amplifiers utilize well known methods of switching power at high frequencies in order to achieve efficiency advantages over analog amplifiers. However, the frequency that digital amplifiers switch at, and the signal processing methods associated therewith, can cause radio frequency interference (hereinafter referred to as “RFI”, or simply “interference”), particularly in consumer electronic equipment incorporating such amplifiers.
In regard to methods for reducing RFI, it is noted that the best possible audio performance measurements for AM radio are significantly lower than the best possible audio performance measurements for digital amplifiers. This fact enables the use of methods that, when reducing RFI, also lower the audio performance measurements without lowering the total system performance, since the AM tuner will be the limiting factor in the overall (i.e., AM tuner+digital amplifier) system performance.
Certain sources of interference will now be discussed with reference to FIGS. 1 and 2. FIG. 1 illustrates the output signal spectrum of a prior art digital amplifier that includes the noise shaper 10 shown in FIG. 2. See, for example, U.S. Pat. No. 5,617,058 to Adrian et al., which is incorporated herein by reference. The output illustrated in FIG. 1 represents the “normal operational mode,” that is, the operational mode in which the interference is not reduced. Note that the level of the signal at any given frequency reflects the amount of radio frequency energy present at the amplifier output.
FIG. 1 illustrates the two main sources of interference present in the AM band between 530 kHz to 1.700 MHz. The first source of RFI is recognized as a distinct component or spike 100 at multiples of the output switching or modulation frequency. Note that Pulse Width Modulation (“PWM”) inherently generates these components 100 at the modulation frequency and at harmonic multiples thereof. Typically, the first three harmonics are in the AM band, since the normal values for the switching frequency are typically 352.8 KHz and 384 KHz, for devices that play back audio CDs, and video surround sound DVDs, respectively. This switching frequency is typically a multiple of the original sample rate, “fs.”
The second source of RFI is noticed as a rise in the noise floor 102 between the modulation harmonics. This interference 102 is generated by the prior art noise-shaper 10 shown in FIG. 2. Noise shaping is used in digital amplifiers to reduce the amount of noise in the audio band (20 Hz to 20 kHz) by increasing the amount of noise outside of the audio band (20 kHz to fs). This creates a high amount of noise power radiated from the switching power stage in the region of 20 kHz to fs. This noise is replicated around all harmonics of the modulation frequency. This noise is also present at frequencies much higher than sample rates (frequencies) that can be seen in FIG. 1. The amount of noise generated between 20 kHz and fs is directly proportional to the amount of noise generated at all higher frequencies. Therefore if the amount of noise between 20 kHz and fs is lowered, the amount of noise at higher frequencies will also be lowered.
Note that the noise shaper 10 does not change the total amount of noise power present, but only redistributes this power across the greater spectrum. To achieve a high level of audio performance, the noise shaper 10 must lower significantly the amount of noise power in the audio band, at the expense of raising the amount of power outside of the audio band.
The relative level of noise power present in the audio band versus outside the audio band is determined by several factors. The largest factor is the order of the noise shaper 10. As shown in FIG. 2, a prior art noise shaper 10 uses a digital Finite Impulse Response (FIR) filter 12 in a feedback loop configuration. Note that the number of previous samples or taps used in this FIR filter 12 determines the order of the noise shaper 10. A larger amount of taps creates a higher order noise shaper, and will push more noise energy outside the audio band and create a higher amount of noise power in the 20 kHz to fs range.